The director of racy movie Fifty Shades Of Grey has said she tells her children it is "a love story they'll never see".

Filmmaker and artist Sam Taylor-Johnson described the erotic romance story as "a dark fairy tale".

The film, based on the book by EL James and expected to hit screens in time for Valentine's Day 2015, tells the story of billionaire Christian Grey who has a passionate relationship with student Anastasia Steele.

When asked what she tells her children about her current role, Taylor-Johnson told The Sunday Times Magazine: "I tell them it's a love story."

Adding: "A love story they'll never see."

The director said: "It's like the world of the Grimm Brothers. I like the transition of this girl and the journey that she goes on... I hope I haven't said too much."

The movie was due to be unveiled in August, but was put back until 2015 having been dogged by difficulties with casting.

The production delay was thought to have been partly as a result of the abrupt exit of star Charlie Hunnam, who left citing his busy schedule and family commitments. He has now been replaced by Jamie Dornan, seen by TV viewers in BBC thriller The Fall.

Dakota Johnson has taken on the role of Anastasia.

Last month it was announced that James's trilogy has sold 100 million copies worldwide.

The series - dubbed ''mummy porn'' - is fast becoming one of the most widely read book series.

The trilogy has been translated into 51 different languages, including Russian, Hebrew, Thai, Serbian, Mongolian, Icelandic, Latvian, Korean, Lithuanian, and Slovak.

As well as selling 27 million copies in the UK and Commonwealth counties, Arrow Books has also sold one million copies or more in Germany, France, Spain, Brazil, and The Netherlands.

Vintage Books sold more than 45 million copies of the trilogy in the US in both print and eBook. Two copies were sold every two seconds during peak sales and it is the fastest selling book series in the history of Random House.

When compared to other series, the figures put the erotic classic on similar figures to Ian Fleming's James Bond series, which was first released in 1953.

But it still has a long way to hit the dizzy heights of JK Rowling's Harry Potter, which has sold a staggering 450 million copies, since the first book was written in 1997.